Transcriptional regulation, which is critical for normal development, is often achieved by the action of transcriptional activators. We study the regulation of the bacteriophage T4 middle promoter PuvsX to investigate how contacts among an activator (the T4 MotA protein), a coactivator (the T4 AsiA protein), and a subunit of RNA polymerase (sigma) lead to activation. During T4 infection the RNA polymerase is modified by the phage. Our transcription experiments have demonstrated that both unmodified and modified polymerase transcribe from PuvsX, but MotA only activates the modified polymerase. The T4 AsiA protein, which associates with sigma, is the modification required for MotA activation. How do MotA, AsiA, and polymerase create an activated complex? Like other middle promoters, PuvsX contains the -10 consensus sequence for binding by sigma. Our footprinting data indicate that both unmodified and T4-modified polymerase contact this -10 region similarly, suggesting that the phage modifications do not affect this sigma binding. This upstream region contains the site where MotA binds to the DNA, centered at -30. Native protein gels indicate that MotA interacts directly with sigma. Our data suggest a model in which sigma makes its usual contact with the -10 region of PuvsX, while the binding of MotA and AsiA to sigma results in stable binding by MotA to the MotA box. We speculate that these protein-protein and protein-DNA contacts then lead to the opening of the DNA so transcription can begin. Surprisingly, the minimal system of MotA, AsiA and otherwise unmodified polymerase, while active for transcription, still cannot recreate the footprint upstream of -40 seen with MotA and the fully modified polymerase. Thus, other modifications to the polymerase, while not required for transcription, may affect the regulation of this promoter.